This invention relates to method and apparatus for producing sheeting having precision patterns where flatness and angular accuracy are important, such as for optical purposes, such as Fresnel lenses incorporating cadioptrics, precise flats, angles and uniform detail, and, more particulary, to method and apparatus for continuously embossing a repeating retroreflecting pattern of fine or precise detail on one surface of sheeting of transparent thermoplastic material to form the sheeting into the desired pattern. Specifically, the techniques are applicable to produce cube-corner type retroreflective sheeting.
Cube-corner type reflectors have been known for many years and many millions have been sold. The phrase "cube-corner" or "trihedral", or "tetrahedron" are art recognized terms for structure consisting of three mutually perpendicular faces, without regard to the size or shape of each face, or the optical axis of the element so provided. Each of the faces can assume a different size and shape relative to the other two, depending upon the angular reflective characteristics desired, and the molding techniques employed. One example of a cube corner type reflector is provided by Stimson U.S. Pat. No. 1,906,655, issued May 2, 1933, wherein there is disclosed a reflex light reflector including an obverse face and a reverse light reflecting face consisting of a plurality of cube corner reflector elements, each having three mutually perpendicular surfaces adapted for total internal reflection of light impinging thereon from the obverse face. Reflectors as taught by the Stimson patent are individually molded and are relatively quite thick and rigid. For many years now, the preferred material for reflectors as taught by the Stimson patent has been methyl methacrylate. Another example of a cube-corner type reflector is the rectangular parallelpiped disclosed in Heasley U.S. Pat. No. 4,073,568.
It long has been desired to obtain the benefits of cube-corner reflectors as used in pavement markers or for automotive purposes, but with the reflector in the form of flexible sheeting. This involves, among other things, a drastic reduction in the size of the cube-corner elements.
Cube-corner type reflectors, to retain their funtionality of reflecting light back generally to its source, require that the three reflective faces be maintained flat and within several minutes of 90.degree. relative to each other; spreads beyond this, or uneveness in the faces, results in significant light spread and a drop in intensity at the location desired.
Prior attempts have been made to produce reflective sheeting wherein the reflective elements are of the cube-corner type. For many years, it was suggested that cube-corner sheeting could not be manufactured using embossing techniques (e.g. Rowland U.S. Pat. No. 3,684,348, Col. 5 ii. 30-42).
More recent attempts at embossing cube-corner sheeting is that of Rowland U.S. Pat. No. 4,244,683, issued Jan. 13, 1981. However, the method and apparatus of Rowland U.S. Pat. No. 4,244,683 are relatively quite complex and only semi-continuous or sequential in nature. Consequently, the Rowland teaching is quite costly to implement, maintain and operate. The operation is slow and the resultant reflective sheeting is quite costly. Moreover, to produce sheeting 48" wide, to be economically feasible, would be prohibitively expensive and complicated using the sequential mold technique of Rowland U.S. Pat. No. 4,244,683.
Also known are other prior techniques for embossing repeating patterns on thermoplastic sheeting, among which other prior techniques are those taught by the following:
Swallow U.S. Pat. No. 2,442,443, issued June 1, 1948; PA1 Hochberg U.S. Pat. No. 3,157,723, issued Nov. 17, 1964; PA1 Kloender U.S. Pat. No. 3,246,365, issued Apr. 19, 1966; PA1 Bergh U.S. Pat. No. 4,097,634, issued June 27, 1978; and PA1 Nyfeler et al. U.S. Pat. No. 4,223,050, issued Sep. 16, 1980.
These other prior techniques do not involve the production of retroreflective sheeting or the precision patterns required for optical purposes. As noted, in order for cube-corner reflective sheeting to be successful, the embossed cube-corner elements must be extremely accurately formed, much more so than is required of the embossed elements of these "other prior techniques", which, therefore, although they may be satisfactory for producing the intended products, may not be adaptable to the production of cube-corner reflective sheeting.
Accordingly, an important object of the present invention is to provide method and apparatus for embossing a repeating retroreflective pattern of cube-corner reflecting elements on one face of sheeting of transparent thermoplastic material, which method and apparatus operate continuously and are greatly simplified with respect to the prior art.
Another important object is to provide such method and apparatus which is less costly than the prior art, in terms of implementation and operation.
A further important object is to provide such method and apparatus enabling continuous production of cube-corner reflective sheeting of reduced cost.
The foregoing and other objects and advantages will appear from the following description of examples of the invention.